Transmission of Nerve Signals Let the fun begin!

Nerve signals Transmission of info from neuron to neuron or neuron to muscle relies on the transmission of “electrical impulses” Electrical impulses: electrochemical messages created by the movement of ions through the nerve cell membrane.

Membrane potential Unequal concentrations of positive ions across the nerve cell membrane neuron membrane 10 Na+ (outside) 5 K+ (inside) – more negative

Resting membrane potential -70mV

Resting membrane potential About -70mV Inside more negative than outside because: Na+/K+ pump: pumps 3 Na+ out and 2 K+ with help of ATP Cell is said to be polarized

Action Potentials A stimulus must be received by the neuron (chemical, mechanical, electrical) Stimulus causes channels on the membrane that are permeable to Na+ to open. Na+ flows down into concentration gradient into the neuron, making the neuron increasingly positive (depolarization) When the cell becomes positive, the Na+ channels close and the K+ channels open, causing K+ to diffuse out of the cell – causes the inside to become negatively polarized again (repolarization). Losing an excess of K+ ions can cause the cell to become hyperpolarized Na+/K+ pump is working the entire time and restores ions to their “original position, which restores membrane potential

Refractory period Time required for a nerve cell to become repolarized Usually 1-10ms Next action potential cannot be conducted until RMP is restored

Conduction in myelinated axons

Threshold potential -50 to -55 mV typically The amount of stimulation which is required to open enough Na+ channels for a complete depolarization to occur “All or nothing” event

Synaptic transmission Once the AP reaches the end of the axon, the impulse must be transmitted between cells at the synapse (small spaces between neurons) Can occur between two neurons or between a neuron and a sensory receptor, gland or muscle

Electrical Synapse Chemical Synapse

Synaptic Transmission When a wave of depolarization reaches the presynaptic membrane, it stimulates the release of neurotransmitter chemicals (from synaptic vesicles) into the cleft These chemicals rapidly pass across the cleft where, they combine with receptor molecules in the postsynaptic membrane The membrane of the receptor then depolarizes and initiates a new AP Although the space is small (~20 nm) transmission slows down across the synapse…more synapses = slower speed

Acetylcholine Neurotransmitter found in the end plates of many nerve cells (esp. neuromuscular junctions) Acts on postsynaptic neurons to open Na channels causing Na to run into the post synaptic neuron causing depolarization However, release of acetylcholine can present a problem…with Na channels open, the postsynaptic neuron would remain in a constant state of depolarization

How can this neuron respond to the next impulse if it never has a chance to recover? Release of an enzyme cholinesterase from the postsynaptic membrane destroys acetylcholine Once destroyed, the sodium channels close and the neuron begins its recovery phase.

Did you know? Many insecticides take advantage of the synapse by blocking cholinesterase. The heart of an insect (unlike the human heart) is totally under nerve control. An insecticide causes the insect’s heart to respond to the nerve message by contracting and never relaxing…therefore resulting in death!

Did you Know? Nerve gases/poisons (such as a poison from the skin of certain South American frogs) compete with acetylcholine for receptor binding sites. By preventing the neurotransmitter from binding to the receptor, it prevents muscular contraction, causing paralysis. Some nerve causes inhibit cholinesterase (therefore no breakdown of acetylcholine) causing continuous muscular contractions (person cannot stop shaking)

Did you know? Dopamine: biological substance that plays a role in sleep, mood, attention, and learning. Imbalances of these transmitters are associated with psychological disturbances and mental illness

Homework Read section 11.2 Answer #1-3 Upcoming Dates: Unit Test  Tuesday